relaytionship
Refers to a type of romantic interaction whereby one of the lovebirds is merely trying to use said romance as a "bridge" or "stepping stone" to either gain access to someone else who's close to his romantic partner (i.e., he figures that he needs to "milk the cow to get to the calf"), or to favorably impress said partner on behalf of a buddy of his who is currently without a snuggle-bug, so that he can then "pass along" said lover to said buddy.
Spoiled-brat Nellie Olson smirkingly confided in Laura Ingalls that she actually didn't give a hoot about Cap Garland for himself; she was actually wanting to pursue Cap's best buddy Almanzo Wilder (whom Laura eventually ended up marrying herself, of course), and so she had befriended Cap merely to allow herself an excuse to get closer to Almanzo. Sounds like a classic relaytionship to me!